I. Reproduction. In an abstract sense, reproduction
serves as the mechanism by which an individual passes genetic information through time.
According to Harold Morowitz, one of my favorite science writers (hes a biochemist),
"an individual is the transient caretaker of the genetic instructions that must
be conveyed down the generations." So, how do plants convey instructions from one
generation to the next?

II. Plants can produce offspring without sex (asexual
reproduction). "A" means without; therefore asexual literally translates into
reproduction without sex. But, exactly what is "sex"? - biologically
"sex" is defined as the process that involves meiosis and fertilization. Thus,
during asexual reproduction there is no production of gametes, no fertilization and hence,
no genetic recombination. Obviously the offspring will be genetically identical to the
parents (=clones). Some points to ponder:

Plants excel at reproduction without sex. Asexual
reproduction is much less common in animals.

Plants practice "safe sex".
In other words,
asexual reproduction is a much safer bet for producing an offspring because an individual
doesn't have to rely on the "chanciness" of the sexual process.

One potential disadvantage.
In theory, asexual
reproduction should limit genetic variation in a population, and hence, would not be a
disadvantage in the long-term evolutionary sense.

Taking advantage of asexual reproduction.
Gardeners have
long taken advantage of the asexual reproduction process in plants. If youve ever
taken a cutting from a plant or grafted a plant, you have, too. Tissue culture is a
high-tech version of the same process. Individuals cells of a plant can be grown
aseptically in culture medium. Initially the cells form an undifferentiated cluster of
cells, called a callus, which is essentially a plant "cancer." These clumps can
be divided up transferring them to other cultures producing literally thousands of
genetically identical cell masses from an original cell. Then, when given the appropriate
conditions and hormones, these calluses will differentiate into an entire new plant! Cool,
eh? Using various techniques (a DNA gun or a bacterial vector) DNA carrying genes of
interest can be inserted into the plant cells and new plants grown. My favorite example is
the tobacco plant that had the gene(s) for firefly luciferase. The plants glowed in the
dark when watered with ATP.

III. Plants have sex,
too.

Sexual Reproduction.
Iinvolves the production of haploid
gametes followed by fertilization to produce a zygote which in turn develops into an adult
that produces more gametes. This process gives rise to genetic variation; offspring are
genetically different than either parent. There are many modifications of this basic
process.

Plants exhibit a unique variation of the generalized sexual
life cycle termed "alternation of generations".
In other words, they alternate between a
diploid and haploid phase in the life cycle. The diploid phase is termed the sporophyte
phase and the haploid phase is the gametophyte phase. The sporophyte produces spores
(microspores or megaspores) by meiosis. The gametophyte produces gametes by mitosis (yes,
mitosis!). The sporophyte and gametophyte phases may be completely separate (as in ferns),
or the sporophyte may be "parasitic" (nutritionally dependent) on the
gametophyte (as in mosses) or the gametophyte may be parasitic on the sporophyte (as in
flowering plants).

Evolution has favored "kinky" sex. In
other words, there has been an trend to increasing the sporophyte phase and
decreasing the gametophyte phase. In mosses the dominant phase is the
gametophyte and the sporophyte is parasitic on it. In ferns, both the
gametophyte and sporophyte are free-living, but the the gametophyte is
relatively small and typically restricted to damp habitats. In
angiosperms, the sporophyte is the dominant phase and the gametophyte is
highly reduced and in the case of the female parasitic on the sporophyte.

V. Flowers are the sex organs of angiosperms (flowering
plants).The flower is the key feature of angiosperms.
It is a complex organ, that produces the male and female gametes. Evolutionarily, the
flower is a short shoot. Thus, a flower is evolved from modified leaves and stem. The
"leafy" nature of the flower can be readily seen in many of the floral
structures. The parts of an "idealized" flower are:

Stem Axis.
The pedicel is the stalk that supports the
flower. The receptacle is the swollen or enlarged terminal portion of the pedicel to which
the other floral organs are attached.

Sepals.
Outermost layer. Collectively called calyx.
Sterile. Often green (photosynthetic), but may be colored or petaloid. Enclose flower in
bud and protect developing inner parts.

Petals.
Collectively termed the corolla, interior to the
sepals. Usually colored, larger than sepals, delicate. The primary function is to attract
pollinators. Pollination - transferring pollen from stamens to carpels. Note: pollination
is different than fertilization.

Stamens (microsporophylls).
Fertile. Collectively called androecium. Male reproductive organ. Interior to petals. Function is to produce pollen
(male gametophyte). Comprised of a(n): (a) anther - microsporangium or pollen sac, usually
2 or 4 chambered, in which pollen is produced. Pollen results from the meiotic division of
a microspore parent cell in the anther; (b) filament - stalk that attaches anther to
receptacle. Check out the proposed evolutionary development in
your text.

You are probably familiar with the term
"pistil" that has traditionally been used to refer to the structure made of
stigma, style and ovary. Botanists prefer to refer this structure as the carpel(s) for
technical reasons, one of which is that carpel highlights the evolutionary origin of these
structures. If you use the term pistil, make sure you spell it correctly - it's not a
weapon. Check out the proposed evolutionary development in your text.

V. Plants like sexual variety - or, there is no such thing
as a "typical" flower. Flowers can be highly modified from the basic floral plan
described above. In fact, finding a flower that looks like the "textbook
diagrams" is somewhat difficult. Most flowers are modified, to a greater or lesser
extent, from the basic pattern.

The fact that all flowers share the
same basic floral plan is good evidence for evolution. If the flowering plants didn't
share a common ancestry, then there is no reason to expect that all flowers would have the
same plan. If different flowering plants were created separately, by individual acts of
creation, then we would expect many different floral designs.

Stephen J. Gould describes the petals
of some orchid flowers as imperfections of nature, meaning that they are
not the "best" or most "perfect" design for solving the problem of pollination.
However, orchids and all organisms are constrained by their evolutionary
history. Thus, these imperfections show the tracks of evolution. Check out
Gould's essay, "The Panda's Thumb."

One common floral modification is to
lack one or more sets of floral organs. If a flower possesses all four sets of floral
parts it is said to be complete. If it lacks one or more it is incomplete. A flower with
both androecium and gynoecium is termed perfect; if missing male or female parts it is
called imperfect and the flowers are considered to be unisexual. Note that by definition a
complete flower must be perfect; but a perfect flower may be incomplete. The terms
monoecious (unisexual male and female flowers on the same individual such as in oak and
birch) and dioecious (unisexual male and female flowers on different individuals such as
in willow and poplars) refer to how flowers are distributed on different plants.

VI. Meiosis occurs in the anther (stamen) and ovule
(carpel). Spore mother cells, also called sporocytes (which are diploid), divide meiotically to produce haploid
spores (microspores - male; megaspore - female). In turn, the spores divide mitotically
and develop into the male and female gametophytes, respectively. The microspores undergo
one division to produce a male gametophyte with two nuclei (tube and generative nuclei). The megaspore
undergoes three mitotic divisions resulting in the female gametophye (also
called the embryo sac) that has eight nuclei (egg, 3 antipodals,
2 synergids, 2 polar nuclei).

VII. Pollen is the male
gametophyte; actually the immature gametophyte. The germinated pollen grain, with its pollen tube and two sperm nuclei
represents the mature male gametophyte. Two sperms develop in the germinating pollen
grain. The pollen tube follows chemical signals on its trip through the stigma and style
to the ovule.

VIII. The female gametophyte is housed in the ovule
(embryo sac). There
may be from one (i.e., cherry) to many (i.e., watermelon) ovules per flower, depending on
the species. In one type of female gametophyte, there are several cells, with a total of 8
nuclei. One nucleus, near the micropyle (opening into ovule), serves as the egg, and two
others in the middle of the gametophyte (also called embryo sac) are called polar nuclei.

IX. Pollination - the plant version of copulation.

Pollination is the transferal of pollen from stamen
(anther) to carpel (stigma)

Plants can have sex with themselves (self pollination) or
with other individuals (cross pollination). There are advantages and disadvantages of
each. There are mechanisms to prevent self fertilization (incompatibility mechanism
involving a gene, S, with several alleles). Pollen will not germinate or the pollen tube
will stop growth if both pollen and stigma have the same S allele.

Plants involve others in their sexual escapades. In other
words, plants rely on a variety of different vectors to accomplish pollen transfer. These
include (a) wind; (b) water; (c) animals (birds, bees, moths, butterflies, giraffes,
etc.); and even (d) fungi (a recent report suggests that a fungus grows over the flower
which attracts flies that eat it thus transferring pollen).

A flower is evolutionarily adapted for its mode of
pollination. For example, wind pollinated flowers have little nectar or odor, reduced
petals and produce copious pollen. There is intimate coevolution between flower and
pollinator.

X. Fertilization. Refers to the fusion of sperm and egg, occurs in
ovule. Pollen germinates on the stigma, the pollen tube grows out and grows through the
style toward ovule. The pollen tube grows into ovule (through a gap in ovule covering
called micropyle) and releases two sperm. One sperm fuses with egg to produce the zygote.
The other sperm fuses with other nuclei to produce a triploid (or pentaploid, depending on
species) cell that produces endosperm. The endosperm is a nutritive tissue that will be
saved primarily for the seed to be used when it is germinating.

XI. Pollination and fertilization are different. This seems obvious but there is a common
misconception that the two terms can be used synonymously

XII. Plant sex is twice as much fun. In other words,
there are two fertilization events (one to produce the embryo and the other to produce
food storage tissue, the endosperm). This process, double fertilization, is unique to
flowering plants.

XIII. Plant embryos are not miniaturized
adults. Embryogenesis refers to the sequence of developmental events that produce the embryo.

XIV. Embryo development in plants is discontinuous. In other words, after the embryo develops a while, it
undergoes a dormant period. In animals, once fertilization occurs, development
of the individual is continuous throughout the life cycle. Dormancy is built in
to allow time for seed/fruit dispersal.

XV. After fertilization, the ovule develops into the seed. A seed is essentially a sleeping baby in a suitcase with his/her lunch. The
three major
parts of a seed are:

Seed coat.
Also called the testa. Is derived from the
integuments (outer layers) of the ovule. Thus, it is derived from maternal tissue (the
only part of the seed that is not part of the new generation). The seed coat is primarily
for protection. The seed coat may have hairs (i.e., cotton) or other appendages. The hilum
is the point where the seed coat detached from the ovary (like our bellybutton). The
micropyle can often been seen (point of entry of the pollen tube into the ovule) near the
hilum. The stalk that connects the seed to the fruit is the funiculus (like our umbilical
cord).

Embryo - which is derived from the zygote.
The embryo has a
region that will give rise to roots (radicle); a region that will produce the shoots
(epicotyl/plumule), a stem axis (hypocotyl) that connects the root end to the shoot end;
and embryonic leaves called the cotyledons or seed leaves. The appearance of these
structures vary with the species. There are two basic patterns; monocot (grass) embryos
and dicot embryos. As the names suggest, perhaps the major difference is that monocots
have a single cotyledon vs. two in dicots. Monocot embryos also have a covering over the
immature leaves (coleoptile) and radicle (coleorhiza).

Food Source.
The germinating seed is initially heterotrophic - it subsists off of its own stored food reserves until it becomes
photosynthetically competent. Thus, it must contain a nutrient source. This nutrient
source can take two forms: (1) endosperm (derived from the fusion of one of the sperm with
the fusion nuclei). Endosperm essentially surrounds the embryo and is common in grasses;
or (2) in other species, the endosperm is reabsorbed during development and stored as
starch, protein or oil in the cotyledons (like beans).

To summarize, seeds contain: (1) an embryo (dicot or
monocot type); (2) with its lunch (in the form of endosperm which may be present or the
endosperm may be reabsorbed and stored in another form in cotyledons); (3) in a suitcase
(seed coat).

XVI. Seeds are an interesting hybrid of maternal and
offspring tissue. The seed coat is derived from the integuments of the ovule.

XVII. The ovary develops into the fruit. A fruit is thus a
ripened, mature ovary and its contents (seeds). Fruits are frequently associated with
accessory structures (i.e., tissues other than those from just the ovary).

XVIII. Whats the difference between a fruit and a
vegetable? Botanically, fruits are derived from the ovary or reproductive parts of the
plants. Vegetables are derived from "vegetative" parts, such as leaves, roots
and stems. Simple, right? So, is a zucchini a fruit or vegetable? How about cauliflower?
or tomato? or squash? Many foods that we call vegetables are actually fruits, and vice
versa. Some "fruits" are really vegetables (such as rhubarb). The problem arises
because "fruit" is commonly used to describe a food that is usually sweet and
eaten as dessert or perhaps salad; whereas vegetable is used to describe a food, usually
green, eaten during the main course of a meal.

XIX. Seeds vs. Fruits.These terms are also often
incorrectly applied (from a strict botanical sense) by the non-botanist. For example,
sunflower seeds and corn grains are actually fruits. Conversely, seeds like coconut, are
often considered to be a fruit. Take-home-lesson: fruits have seeds, vegetables do
not...the seeds are inside the fruit.

XX. The function of fruits (and seeds) is to send
"the kids off to college". Fruits protect the seeds and function in dispersal.
They can be disseminated by wind, water, animals, etc. Note: fruits do not provide
nourishment for the germinating seedling - they provide nourishment and an
entincement for the dispersal agent.

XXI. The mathematics of plant sex. 1 pollen grain/ovule;
2 sperms/ovule. How many ovules, sperms, eggs, ovaries, flowers were necessary to produce
a watermelon? or a cherry?

XXII. Seed germination.
(not on exam) The first stage is water uptake
(imbibition). This is followed by rapid swelling of the seed and then initiation of
germination events. The first real sign of germination is the appearance of the radicle.
In grains, this process is mediated by the plant hormone gibberellic acid that is released
by the embryo that stimulates the release of amylase from aleurone cells.